Electrode mass for self-baking electrodes



Patented May 15, 1928.

I UNITED STATES PATENT OFFICE.

cam. WILHELM sonnnnnno, or 08130, NonwAY, assrcnon. To our Nonsxn Amara snnsxan ron nnnxrnoxnmsx mnusrnr or nonwnr, or can, nonwar.

nnnc'rnonn mes FOB SELF-BAKING nnncrnonns.

ll 'o Drawing. Application filed July i2, 192*, Serial No. 727,587, and in Norway July 25, 1823. i

The present invention relates to 'the' socalled continuous electrodes, which are baked in the electric furnace in which they are used and the object of the invention is to provide an improved electrode mass from which an electrode is obtained hav ng first class mechanical and electric qualit1es. The usual carbon electrodes for electrothermic use are, as'is known, made by mixing anthracite, previously heated to a red heat, cinders and other carbon materials with a binder, comprising pitch with some oil, or tar and pitch, to form a mass-which is pressed or tamped into moulds. The electrodes thus formed are then baked in special baking furnaces where they are slowly heated until the binder is ooked, usually to least possible amount of volatile constituents. The less volatile constituents there are contained in the electrode mass, the

higher will be the specific gravity of the electrode.

2. The moulding of the electrode must be effected by means of high pressures or powerful blows whereby the particles of the 65 as it is termed be able to take pressing?" mass are pressed together and the air con tained is driven out.

The method which is usually employed and which is considered to be the best is to press the electrodes in hydraulic presses, em-

ploying pressures of about 300 kg. per square centimeter. The electrode mass is subjected to this high pressure for a long time and thereby the particles of the mass are displaced relatively to each-other and the air is driven out.

Also by tamping one has been able to make good electrodes. The process then consists in subjecting the electrode mass to a tamping b means of a ram-like tamping apparatus. 0th with regard to the transportation and the baking of the raw elec trodes and to. make the mechanical treatment of the mass efiective the electrode mass must have a definite consistency. It must I and take tamping.

,mass which is used in the production of the so-called selfbaking electrodes must have a consistency quite difl'erent'from usual mass to givea satisfactory result, it being-usually impossible to employ high pressures for press1ng the mass, or powerful tamping. Qnly a llght tamping may be used,'effected eithenmanually or by means of small pneumatic tamping devices; If it is attempted to tamp usual electrode mass in this way it will not be possible to forcethe mass sulficiently together and an electrode results Withlower specific gravity than usual aloe-- trodes, and the coherence and electric conductivity will be bad. This has the effect,

that the electrodes stand less current density than first class electrodes and may in many cases lead to breakage, the electrodes not being strong enou h to stand the strains to which they will e subjected. This behaviour is especially noticeable in the production of se fbaking electrodes which are provided with an ironor metal armature. Such electrodes generall consist of an iron mantle with inwardly irected' radial longitudinal ribs and this armature encloses the electrode mass and determines its form. In the lower part of the armature facing the furnace the electrode'mass will. be ready baked, in the upper part it is entirely raw.

The ribs transmit the current from the electrode holder to the baked part of the electrode and hold this tight for which reason they are provided with projections or the likewhich prevent the hard baked electrode from moving relatively to the ribs. Such a continuous electrode is generally4 to 7 meters long.

Even if in using ordinary electrode mass for suchelectrodes it is possible by extremely accurate tamping to reach the right specific gravity of the electrode, the-resulting electrodes will .nevertheless not be so reliable as desired. When subjected to special strains electrode breakage may occur. The electrode divides up in pieces of 30 to 90 centimeters length which fall into the furnace when the iron armature is melted away.

2 emons The reason for this is that the electrode mass during the tamping gradually hardens as the carbon particles are worked together.

The resulting hard non-electric mass is fixed the armature is subjected to a strainwhich may result in a division of the electrode into pieces whose length varies with the electrode diameter and temperature conditions.

I have now found that the difficulties heretofore pointed out may be overcome by iving the mass a consistency with suflicienty low viscosity to allow the mass to settle during the heating in the electrode. To reach the right specific gravity and corresponding electric conductivit and mechanical qualities the mass must uring the. bak-vv ing make a slow settling possible. The baking takes place in a mass subjected to pressure fiom the column of mass above. I obtain a suitable consistency by. increaslng the content of liquid constituents in the mass. The ordinary electrode mass for large electrodes generally contains a mixture of tar and pitch (for instance in the proportion 3 tar: 1 pitch) constituting 10 to 11% of the mass and is then easily tamped to a hard mass. If now the amount of tar-pitch is increased the mass will become more sticky at the tamping temperature and consequently diflicult to treat with the tamping machine. At 18 to 22% tar-pitch it cannot be tamped at all. The treatment of one part of the surface of the mass with a machine I for instance a pneumatic tamper with point only results in pressing down this part of the surface and raising the others. The consistency of the mass is that of a dough an the object of the tamping is in this case not to beat the mass together to attain a high specific gravity as in the case of ordinary pressed or tamped electrodes but only to bring the single portions of massinto intimate contact with each other and remove greater air bubbles. 1

My. investigations have shown that such a mass has at the lower part of the continuous electrodes a considerably higher specific gravity than newly tampedmass in the upper part of the electrode. Samples of a ready baked electrode taken out from the lower end of a selfbaking electrode have the same specific gravity as the. best pressed electrodes of the trade. This shows that a slow settling of the mass takes place while it is in the electrode subjected to heat from the smelting furnace and charged with the weight of the mass above.

' The difference between tampable and non-tampable mass is very apparent. Tampable mass subjected to a series of blows from a tamper becomes harder at each blow. At last the tamper strikes the mass with a soundas if striking a perfectly-solid, nonelastic body. In a non-tampable mass such solidity is never obtained. In order to make clear how the change in consistency takes place when increasing the amount of liquid binder I have carried out comparative measurements of viscosity in a series of mixtures of finely pulverized anthracite and a liquid binder. The viscosity was determined by noting the number of seconds required for a rod of definite Weight to move through a layer of mass of definite thickness. The results are given below:

At 24% hinder the movement of the rod was not noticeable.

At 25% binderthe movement of the rod was noticeable, but the time required was more than 1 hour.

At 26% binder themovement of the rod was a little quicker, but the time required was still more than 1 hour.

At 27% binder time still more than 1 hour.

Amount of binder: 27.5%, 28%, 28.5%, 29%, 30 32%.

Time' required: 760, 100, 65,- 58, 39, 20

will be seen, that the curve makes a sharp turning at 27.5 to 28% binder. This sudden change in the movability of the mass 9. parently corresponds to such a contentof liquid that the solid carbon particles of the mass are no longer directly in touch with each other, but slide on the lubricant, the binders used in electrode production being at the sametime excellent lubricants.

The absolute percentage at which the mass ceases to be tampable depends on the amount of fine particles in the electrode mass and on the nature of the fines, and can therefore not be determined beforehand. In the investigations referred to above the turning point of the curve was found to be at 27.5% binder. This is of course only the case in a mixture of definite fines with a definite binder. If the quality or fineness of the fines are altered the turning point will become displaced. Under all circumloo binders. The coarse particles require very little I binder to .become moistened. The pieces are in the mass completely surrounded by fines on all sides and the consistency of the fines determines that of the mass. Practical experiments on a great scale now show, that the characteristic change in consistency by employment of the same material as in the experiments previously referred to, but with about of coarse particles takes place at about 18% binder, and at 20% the mass is very movable. These figures are in good accordance with those found above it being considered that of the mass requires very little binder. If more coarse particles are employed the change takes place at a lower content of binder. The mass is always treated in warm condition, generally at tempera tures ranging from 60 to 100 C. The mixture of binders used in practice is then liqgid.

y making the content-0f volatile con stituents and binder in the mass sufficiently high ta-mping of the mass may be avoided altogether and the production of the continuous electrodes more takes the character of a casting. But also in such cases an excellent conductivity and specific gravity is attained in the finished electrodes, this being due to the exceedingly favorable baking conditions.

As mentioned above the iron mantle moves slowly downwards together with the electrode mass towards the crater of the furnace. the temperature of the mass rising slowly from. about 60 C. to a bright red heat at the point of the electrode. At about 200 C. the first distillation of the most volatile constituents of the mass begins and these escape in the form of vapor. At about 700 C. the binders are transformed to highmolecular carbon compounds which are slowly transformed to a gradually harder coke. Thereby the porosity of the electrode is increased. The raw mass durin the tamping still contains some air, but uring the storing in the electrode a part of the air escapes and the porosity sinks to about 5%. In the completely baked electrode it is about 15 to 20%. I have found that during the distillation and coking the excess of binder will together with the distillation products try to escape downwards through the already baked and consequently most porous part of the electrode, but as just this part is heated to a red heat new amounts of carhon-hydrogen compounds will here continually carbon particles consequently become further bound together, and the weight and conductivity "of the electrode increases.

The foregoing detailed description has been given 'for clearness of understanding be reduced, coke being formed and deposited in the pores of the electrode. The

only and no undue limitation should be deduced therefrom but the appended claims should be construed as broadly as permissible in view of the prior art.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrode mass for use in forming continuous electrodes, containing an excess of binder in proportions suiiiciently high to cause the mass in warm condition to settle in the electrode.

2. An electrode mass for use in forming continuous electrodes, in which. the proportion of binder content is sufficiently high to render the mass elastic and non-tampable.

3. An electrode mass for use in forming continuous electrodes, in which the proportion of binder content is sufiiciently high to prevent reduction of the total volume of the mass under a tamping action.

4. An electrode mass for use in forming continuous electrodes, in which the proportion of binder content is sufficiently high to render the mass liquid.

5. An electrode mass for use in forming continuous electrodes, in which the binder content forms at least 14% of the mass.

6. The process of forming an electrode baked in the furnace in which it is used, and includingan enclosing mantle comprising the steps of forming an electrode mass having a binder content suiiiciently high to render the mass elastic and non-tampable and then casting the mass within the enclosing mantle of the electrode.

7. The process of forming an electrode baked in the furnace in which it is used and including an enclosing mantle comprising the steps of forming an electrode mass having a binder content sufficiently high to render the mass liquid, and then casting the mass within the enclosing mantle of the electrode.

8. The process of forming an electrode baked in the furnace in which it is used. and including an enclosing mantle comprising the steps of forming an electrode mass having a binder content of at least 14% of the mass and then casting the mass within the enclosing mantle of the electrode.

9. An electric furnace electrode of the kind which is baked during its use in any electric furnace characterized by a baked portion at the operating end and by soft electrodepaste at the feeding end, said paste being of such consistency as automatically to exert sufiicient pressure by its own weight to assure such compa tness of the electrode materials throughout the baked portion of the electrodes as to afford physical and electrical characteristics of a pre-baked electrode.

CARL WILHELM SODERBERG. 

